This invention relates to devices for slowing the rate of a stream of data. Some communication systems transmit data in extremely rapid bursts, either for security reasons or for purposes of time-compression multiplexing. A receiver of the time-compressed data stream may not be able to process it at the high transmission rate. Although electronic equipment could be designed to process received data at high speeds, such equipment is complex and expensive. Accordingly, the cost of receiver equipment for receiving high-speed transmissions would be considerably reduced if the incoming data rate could be reduced by a factor of ten to twenty.
Digital data are typically transmitted by modulating a high-frequency carrier signal. One widely used modulation technique is quadrature phase-shift keying (QPSK), in which digital data are coded as selected phase shifts in the carrier signal. The carrier may assume any of four quadrature relative phases, and is subject to a phase shift for each bit of data transmitted. In one typical application, the signal transmission rate within a burst may be in the order of 120 Mbps (megabits per second), and each burst may include around 1,000 bits, in an interval of about 8-10 microseconds. Any data rate expansion technique must preserve the phase of the original signal for subsequent demodulation.
Surface acoustic wave devices have been proposed for use in time-expanding or time-compressing applications. For example, U.S. Pat. No. 4,356,353 issued in the names of Eng et al. discloses time "companding" apparatus for use in a time-compression multiplexing system. The basic technique disclosed by Eng et al. is to process a signal with a Fourier transform circuit, a multiplication circuit, and an inverse Fourier transform circuit. As is well known, Fourier transformation of a time-varying signal is equivalent to transformation from the time domain to the frequency domain. The Fourier transform circuits in the Eng et al. patent are implemented as surface acoustic wave (SAW) devices in the form of linear dispersive filters. By appropriate selection of the filter parameters, the apparatus performs a desired degree of time expansion or compression.
The Eng et al. technique requires the use of two convolver circuits and five multiplier circuits. The convolvers and four of the multipliers, or mixers, all employ SAW devices in the form of chirp filters. One embodiment of the present invention is directed to an improved technique of the same general type as that disclosed by Eng et al. Other embodiments of the invention achieve time expansion by somewhat different but related approaches.